The present invention relates to dc motor controllers and, more particularly, to control systems for dc motors having separately excited armature and field windings.
Heavy duty material handling vehicles provided with electric motors typically use a lead-acid battery that can weigh many thousands of pounds. Besides providing the energy source to the vehicle, in many instances the battery also provides vehicle counterbalance.
The ratio of the load weight to the gross unloaded vehicle weight of industrial lift trucks is extremely important. For example, if an unladen vehicle weighs 12,000 lbs, and the maximum load weight it can carry is 4,000 lbs, then the gross unladen/laden weight may vary from as little as 12,000 to as much as 16,000 lbs. This represents a change of 33% in motor torque requirements. Moreover, the vehicle must be able to maneuver on loading ramps, further increasing the motor torque requirements. For these and other reasons, a control system capable of extracting precise and efficient work from the vehicle is desirable.
The main motive element of this type of vehicle, referred to as the traction system, usually consists of a series-wound dc motor coupled to a gear reducer and drive wheel.
The rotational direction of the series-wound dc motor is controlled by the polarity orientation of the field winding with respect to the armature. Generally, the field winding orientation is controlled through a pair of contactors, such that when power is applied across the field-armature combination, the motor is caused to rotate in the desired direction.
The series-wound dc motor, heretofore used extensively in industrial lift trucks, features one very important characteristic: it has high torque at zero speed. This is extremely useful in providing the necessary starting torque.
Typically, the field-armature combination is controlled as a single unit. Motor speed regulation is most often achieved through voltage switching utilizing such power semiconductor technologies as silicon-controlled-rectifiers (SCRs). The voltage drop associated with the SCR as well its duty cycle limit impose a speed limit on the motor.
However, a series dc motor may operate only along its characteristic commutation curve limit. Since changing torque loading arises from variations in load capacities, travel path conditions and grade variations, motor speed variations occur.
With the proper controls, the use of a shunt-wound dc motor under independent field and armature control can provide distinct advantages over conventional series-wound dc motors for lift truck applications.
U.S. Pat. No. 4,079,301 issued to Johnson, III discloses a dc motor control circuit having separately excited armature and field windings. The control circuit is operable in both the constant torque and constant horsepower modes. The transfer characteristics of the circuit provide high gain at low frequencies and low gain at higher frequencies. The circuit can further reduce the gain at low frequencies when motor operation switches from the constant torque mode to the constant horsepower mode.
U.S. Pat. No. 5,070,283, issued to the present applicant and hereby incorporated by reference, discloses a control method that provides a shunt-wound dc motor with the ability to simulate a series-wound dc motor, hence developing the necessary starting torque. Feedback is provided by an encoder, connected to the armature of the motor, for indicating motor speed.
Unfortunately, monitoring motor speed is an indirect method of determining the speed of the vehicle itself. Moreover, since the speed of the traction motor is not always linearly proportional to the actual speed of the vehicle (e.g., when turning), attempting to control the motor based solely on the speed thereof is not necessarily the most accurate vehicle speed control method.
Moreover, connecting or attaching an encoder to the armature of the motor is often problematical. Not only must space considerations be taken into account, but heat dissipation techniques must be employed.
It would be advantageous to provide a system for controlling a motor based on data representative of motor speed, but not to require motor speed measurement at or near the motor itself.
It would also be advantageous to provide a system for controlling a motor that is based on direct speed measurement of the wheel(s) driven by that motor.
It would also be advantageous to provide such a system in which the motor can be controlled by a decoupling controller.
It would still further be advantageous to provide such a system in which the decoupling controller is achieved using software.